Superconducting tunneling as a probe of sputtered oxide barriers

The tunneling properties of sputtered oxide barriers were studied in Pb/oxide/ferromagnet junctions. The initial oxide/ferromagnet bilayer was made without breaking vacuum. The bilayer was exposed to atmosphere before the deposition of a Pb counterelectrode in a separate vacuum chamber. I-V curves and conductance measurements at 1.5 K confirmed the presence of single-step, elastic tunneling in these structures. Separate experiments involving gold bottom electrodes, variation of exposure times in air between bilayer and Pb depositions, and plasma oxidation proved that the oxide layer is permeable, allowing for oxidation of the ferromagnetic base electrode. This revealed that the tunneling channels were due to the contribution of small-area junctions at thermally oxidized "pinhole" sites on the bottom electrode. (C) 1999 American Institute of Physics. [S0003-6951(99)03327-6].